1. Field of the Invention
The present invention relates to a flow sensor that is arranged, for example, on an intake pipe of an internal combustion engine so as to be used as an air flow sensor (AFS), for detecting the flow rate of fluid by electrically detecting the amount of heat transmitted to the fluid from a heating element arranged therein. In particular, the invention relates to a flow sensor provided with a so-called ratiometric output circuit that outputs a voltage (or current) proportional to a voltage value of an A/D conversion reference power supply of a fuel injection control unit in an internal combustion engine. More specifically, the invention relates to a novel technical improvement of a calculation circuit in such a flow sensor that calculates the voltage value (reference voltage) of the A/D conversion reference power supply of the fuel injection control unit from a voltage supplied to a sensor element (e.g., a temperature sensor) formed integral with the flow sensor.
2. Description of the Related Art
Although in conventional flow sensors, there are a variety of output forms of flow rate detection signals such as voltage, current, frequency, etc., a flow rate detection signal is used, for example, for the fuel injection control of an internal combustion engine.
A fuel injection control unit connected to a flow sensor performs fuel injection control in accordance with the output value of a flow rate detection signal from the flow sensor, but in case where the output form of the flow sensor is a voltage or current, in order to reduce errors occurring upon A/D conversion in the fuel injection control unit, very high accuracy is required of the voltage value of an A/D conversion reference power supply in the fuel injection control unit.
Accordingly, the flow sensor has a so-called ratiometric output function, and is constructed so as to output a voltage proportional to the voltage value of the A/D conversion reference power supply supplied from the fuel injection control unit (see, for example, a first patent document: Japanese patent application laid-open No. H9-203651).
In this case, the flow sensor is provided with a special or dedicated terminal which serves to supply the voltage of the A/D conversion reference power supply from the fuel injection control unit, so that a ratiometric output is formed from the A/D conversion reference power supply.
In addition, in a recent fuel injection control system, to rationalize the assembling work of a sensor element (e.g., a temperature sensor) in a flow sensor, there has also been proposed a technique in which a temperature sensor is formed integral with the flow sensor (see, for example, a second patent document: Japanese patent application laid-open No. 2001-153701). In this case, a voltage from an A/D conversion reference power supply of a fuel injection control unit is supplied to the temperature sensor (e.g., thermistor) through a resistor and a connector terminal.
In conventional flow sensors, there is the following problem. That is, if a ratiometric output function is provided as in the first patent document, a special or dedicated terminal to supply the voltage of an A/D conversion reference power supply from a fuel injection control unit is needed, and hence it is impossible to achieve cost and size reduction.